In semiconductor fabrication, semiconductor memories are produced that have a multiplicity of memory cells arranged in a memory cell array. A distinction is made between volatile and nonvolatile semiconductor memories. Nonvolatile semiconductor memories, which store stored information items even after the voltage supply has been turned off, may be formed as phase change memories. Phase change memories have a storage medium, which can assume two different states of different levels of electrical conductivity and undergoes a phase transition between these two states. The different conductivity of the storage medium can be utilized in phase change memories in order to store digital information items. By way of example, a layer made from a phase change medium is arranged between two electrodes to which an external voltage is applied. On the basis of the magnitude of the current flow through the electrodes and the phase change medium, it is possible to determine the state of matter of the phase change medium. Furthermore, by means of higher currents, which heat the phase change medium to higher temperatures above the crystallization temperature or melting point, the state of the storage medium can be altered and a digital information item can thus be overwritten.
Furthermore, further concepts exist for storage media that, on account of other properties, for example magnetic properties or their ferroelectric polarizability, can assume different states in which the respective electrical conductivity of the storage medium differs to an extent that suffices to represent two different digital information items.
In an integrated semiconductor circuit, the memory state of an individual memory cell can only be altered electrically, that is to say by a current flowing through the storage medium or in the vicinity thereof, or by a voltage applied to the storage medium. The higher the current required for changing the state of the storage medium and thus for rewriting the digital information, the higher the energy consumption of the integrated semiconductor memory. Particularly in the case of phase change media and other storage media whose state is dependent on temperature, the currents for heating to the phase transition temperatures or melting points require a considerable amount of energy.
The temperature increases required for programming the memory cells can only be achieved by high programming currents. Memory components comprising a phase change medium, which requires these high programming currents, cannot readily be integrated together with other components such as selection transistors into a single semiconductor circuit if the other components are intended to be dimensioned according to the smallest possible feature size. In present-day memory circuits, permissible maximum currents of CMOS transistors are approximately 0.5 mA per micrometer of channel width.
Moreover, there is the problem that in the case of high currents for rewriting a digital information item, only very few memory cells can be overwritten simultaneously without exceeding the maximum permissible current density in partial regions of the semiconductor circuit. Therefore, the programming currents that are still very high at present restrict the degree of parallelism of memory operation.